The invention relates to helical antennas and, in particular to helical antennas for use in wireless communications, having an improved impedance matching section.
Antennas are a basic component of all systems using electromagnetic radiation to transmit and receive information. Many types of antennas exist, ranging from a single monopole antenna to complicated antenna arrays. Each type of antenna has its own strengths and weaknesses, making the selection of the appropriate type of antenna for a system dependent upon the performance and frequency range requirements of the system.
In some systems, the frequency range and properties of axial mode helical antennas are beneficial. Helical antennas typically have a radiating element (an electrical conductor of some sort) in the shape of a helix, which is attached to some sort of back plate, and which is connected to a signal generator, or radio. Helical antennas, unlike other types of antennas, emit and respond to electromagnetic radiation with a circular polarization. This polarization can be either left- or right-hand polarization depending upon the orientation of the helically shaped radiating element. This characteristic can help reduce problems due to multipath signals because a reflected signal typically will be the opposite polarization from the original. This cross polarization will typically produce up to a 20 dB attenuation in the reflected signal. Due to this drop in signal strength the multipath signal will typically be treated as noise and disregarded.
Typically, helical antennas are employed in systems involving satellite communication with Earth-based stations. The frequency range and other attributes of a helical antenna depend upon the physical characteristics of the antenna, such as the radius of the helix, the distance between turns on the helix, and the pitch angle of the helix. Ideally, the length of a single turn of the helix should be around the peak wavelength in which the antenna transmits and receives. Furthermore, the gain of the antenna is roughly proportional to the overall length of the antenna, while the beam width of the antenna is roughly inversely proportional. A typical open-air axial mode helical antenna for the 2.4 GHz Industrial, Scientific, Medical frequency band (hereinafter xe2x80x9cISM bandxe2x80x9d) will be around 1.5 inches (3.81 cm) in diameter with the length dependant upon desired gain and beam width. The exact frequency range for the ISM band varies from county to country, but is typically about 2.4 GHz to 2.5 GHz.
It is known in the art that these dimensions can be effectively reduced by loading the helical antenna with a dielectric substance at the center of the radiating element. In this manner, an effective helical antenna for the ISM band can be around 1 inch (2.54 cm) in diameter and with the length varying based on the desired beam width and gain, depending upon the dielectric constant of the substance used. This reduction in the size of the antenna makes helical antennas a feasible option in a wider range of wireless communications environments.
Typically, the electrical impedance of the helical antenna""s radiating element will differ from the electrical impedance of the electrical connector for a supply network (i.e., connector for a signal generator, or a radio, etc.) to be used with the antenna. To accommodate this difference in electrical impedance, the helical antenna must typically include an impedance matching network at an interface between the radiating element and the electrical connector for the supply network in order to prevent signal reflection, or loss, at that interface. Matching networks are well known in the art, as are other solutions to the impedance matching problem. A typical matching network may consist of an electrical circuit printed on a flexible substrate mounted near the back plate of the helical antenna. A first end of this matching network is connected to the antenna""s radiating element and matches the electrical impedance of the antenna""s radiating element at the first end. A second end of the matching network is connected to the electrical connector for the supply network and matches the electrical impedance of the connector at the second end.
An alternative to the above matching network is a matching section comprising a tapered piece of metal that has a length of approximately one-quarter of the antenna""s operative wavelength. This matching section tapers from approximately the width of the radiating element, at the end connected to the radiating element, to a width that provides an appropriate impedance for the particular electrical connector for the supply network, at the end connected to the electrical connector. This type of impedance matching section is typically affixed near the proximal end of the cylinder supporting the helix. The prior art affixes the matching section in a manner that leaves the matching section vulnerable to stresses exerted by the connector for the supply network as well as by the radiating element (including any stress associated with a structure supporting the radiating element). Tension exerted on the matching section by the connector for the supply network and by the radiating element can often be sufficient to distort the matching section""s shape. This distortion can render the matching section inoperative or less effective, resulting in signal reflection and loss at the interface with the antenna""s radiating element and/or at the interface with the electrical connector for the supply network. Thus, a need exists for an improved matching section that is less susceptible to distortion from stress exerted by the connector for the supply network or by radiating element.
The present invention relates to a helical antenna in which a radiating element, formed into a helix, rests on an elongate core and connects to an impedance matching section at the proximal end of the elongate core for matching the impedance of the radiating element to the impedance of an electrical connector that serves to connect the antenna to a radio. The elongate core is mounted to a back plate, which is adapted for mounting the antenna to a support structure. The impedance matching section is seated in a helical groove in an exterior major surface of the elongate core adjacent to the proximal end of the elongate core, thereby providing additional anchoring and structural support to the impedance matching section. This additional structural support renders the improved matching section less susceptible to mechanical stress imparted by the connector for the supply network or by the cylinder and the helix.
Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.